First time using my offset and fuel consumption questions

So I recently got a Oklahoma Joe Longhorn and did the following mods to it:

gasket tape around the firebox door and main cooking chamber door. reduced smoke escape by 98%. There is still some escape b/c the door isnt heavy enough to put more pressure now that the is some tape on the edges. Fixable by putting some weights on it.

added high-heat sealant to the all joints around the firebox and around the chimney. Reduced smoke escape by 100%.

added 90" angled tube to the inside of the chimney to bring the vent down to the grate level.

raised the level of the fire box grate by 3" to give more room for ash.

build a 12"x12"x12" charcoal box out of mesh steel.

I did my first smoke on it yesterday, 10lbs full packer brisket and here is what i noticed.

started with full basket of kingsford charcoal briquettes (around 5lbs).

i monitored temp at grate level right in front of the brisket.

could not maintain 250F for more than 1 hour without having to add more charcoal. - When needed to add more, I would light up a chimney full and add it to the firebox when its starting to get white. I did this pretty every 90 mins over 10 hours.

temps fluctuated from 220 to 260 for the entire cool.

I cleaned out the ash once i saw it was getting to be too much. did this 2-3 times during the day.

I ended up using around 25lbs of charcoal and around 3lbs of cherry wood chunks for the entire cook.

For those with offsets like mine of similar size, are you using this much charcoal? Is this normal? Not sure what else I could do to optimize charcoal usage.

Hello. Welcome. I see this is your first post. Please take some time and swing over to Roll Call and introduce yourself so that we may give you a proper "Hello". All info you can provide us with such as smoker type, location and so on will help us answer any questions you may have, and pictures help a bunch. As for your question:

I do not have one now but I used an offset for MANY years. Some I built from 1/4" pipe and a couple cheap Chargill. I did not even bother to seal the Chargrills. I have RARELY used more than a 5 lb. bag on a 12 hour smoke. I used lump wood charcoal and never used briquettes but the process is the same. You may have an air flow problem but from reading your post I think you are using the smoker in the wrong way. You mentioned adding fuel to the smoker. Lighting a chimney and when the coals were white adding the to the fire. That is the correct way to GRILL. In a smoker what you want to do is light a chimney, get it white hot and then add unlit charcoal on top. You then use air flow to control how fast the unlit charcoal ignites. That way you control temp. If the temp starts to drop open the vent and more charcoal will ignite, raising the temp..

Also don't get too stuck on 225f.. 225 is the "ultimate" but not totally necessary. I actually let mine float between 225-300 without worry. I do try not to go above 300 for a long period or below 225 for a long period (30-45 minutes ). Have fun. Keep Smokin!

In my offset, I start with about 2/3 basket of Royal Oak lump and start it with about 2/3 chimney of Royal Oak briqs. When the lump is fully involved, I add a couple of splits. The lump gives a much better bed of coals than briqs and doesn't leave nearly as much ash. When the CC gets to cooking temp, I add another split and my flavor chunks and put my meat in. My smoker likes to run at 260-275*. When it gets down to around 250*, I add a couple of pre-heated splits, always keeping a good bed of coals. The coal bed is the trick to good heat retention. Pre-heating splits allows them to ignite very quickly and helps to keep the heat up faster.

I hope this helps you with your heat and fuel management. It works good for me. Joe